Vinylidene chloride interpolymers are well known in the prior art. Such polymers are also well known to be thermally unstable, which means that upon exposure to desirable processing temperatures such polymers tend to generate undesirable physical properties such as an increased level of carbonaceous material contamination (i.e., contamination by a variety of dark and intensely-colored species), evolve hydrogen chloride, and have poor extrudability, e.g., discoloration.
In an effort to improve the extrudability of polymeric compositions comprising vinylidene chloride interpolymers, such compositions are fabricated, in either powder or pellet form, mainly from vinylidene chloride interpolymers and an adequate amount of modifiers such as stabilizers, plasticizers, etc. When using no modifiers with the vinylidene chloride interpolymer, the high frictional and viscous forces on the vinylidene chloride interpolymer result in the vinylidene chloride interpolymer being subject to thermal decomposition, e.g., having carbonaceous material contamination, discoloration, or hydrogen chloride evolvement due to the close proximation of the processing temperature to the vinylidene chloride interpolymer's thermal decomposition point. Moreover, the decomposed interpolymer may generate an undesirable level of carbonaceous material contamination in the extrudate which, if the carbonaceous material extends through the thickness of the extrudate article, could have a deleterious effect upon the gas permeability of the extrudate.
However, in order to industrially extrude and process vinylidene chloride interpolymers without carbonaceous material contamination, discoloration, or hydrogen chloride evolvement in the product, a relatively large amount of stabilizer and plasticizer would inevitably have to be incorporated into the resin. The larger amount of plasticizer reduces the melt viscosity, thereby reducing the processing temperature which improves thermal stability: and the larger amount of stabilizer improves the thermal stability of the polymeric composition. However, the relatively large amount of a stabilizer and plasticizer increases the permeability of the extrudate to atmospheric gases.
In the past, the practice has been to extrude the vinylidene chloride interpolymer directly from the powdered form in which it is recovered. Because of the convenience of shipping and handling, it is desirable to form the vinylidene chloride interpolymer into pellets prior to final extrusion. As the demand for pellets has increased, the demand has increased for a pellet which can withstand the myriad processing conditions to which powdered resins are subjected.
While pellets of thermally sensitive polymeric compositions may be an advantageous form from which to fabricate articles, such pellets of thermally sensitive polymeric compositions are particularly difficult to extrude. Pellet formation requires an exposure of the thermoplastic composition to heat prior to being extruded into an article. This additional heat history is believed to make the pellet form of the polymeric composition more susceptible to thermal instability. Consequently, additive packages which improve the thermal stability of thermally sensitive polymeric compositions in powder form do not necessarily improve the thermal stability of thermally sensitive polymeric compositions in pellet form.
Although satisfactorily extrudable for a period, it has been found that attempts to extrude vinylidene chloride interpolymer pellets over long periods on certain extrusion equipment have also proven unsatisfactory due to an undesirable level of carbonaceous material contamination, increased discoloration, or higher hydrogen chloride in the extrudate. Additive packages which improve the thermal stability of thermally sensitive polymeric compositions in powder form do not necessarily improve the thermal stability of thermally sensitive polymeric compositions in pellet form.
It is desirable to produce a polymeric composition which, if subjected to desirable elevated processing temperatures, generates a reduced level of carbonaceous material contamination, evolves less hydrogen chloride, or has better extrudability, e.g., color, than vinylidene chloride interpolymer alone. Moreover, when the polymeric composition is processed at desirable temperatures the extrudate also possesses a reduced level of carbonaceous material contamination, less discoloration, or less hydrogen chloride evolvement than an extrudate formed from vinylidene chloride interpolymer alone. It is to this goal that the present invention is directed.